This document provides an overview of diffusion and osmosis in plants. It defines diffusion as the movement of molecules from an area of higher concentration to lower concentration, which can occur actively through external energy or passively without energy. Osmosis is defined as the diffusion of water across a semi-permeable membrane from an area of lower solute concentration to higher. The document outlines the factors that affect diffusion and osmosis rates, the different types of osmosis, and the importance of these processes for water and mineral absorption and transport in plants. Measurement techniques for water potential and status are also discussed.

1. Fundamental of Crop Physiology
Dr. Shailendra Bhalawe
Assistant Professor
College of Agriculture Balaghat
Jawaharlal Nehru Krashi Vishwavidyalaya Jabalpur
(M.P.)

2. Chapter 03: Diffusion
Diffusion: The movement of free molecules of gas, liquids, and solids from the
region of higher concentration to lower concentration due to internal or external forces
is called Diffusion.
Movement---- Active movement-----External energy
Movement ----Passive movement----No need energy
 Solvent + Solute ------Solution
Eg. Water + Salt----Solution
Diffusion difference- gradient.
Ex. Diffusion blowing of wind,
Ex. Dispersal of good smell of agarbathis in a room,
Ex. Dissolution of sugar in water.
Such as a diffusion of molecules in any region continues till it spread thought the
available area. The movement of molecules depends upon the internal kinetic
energy. The molecule in the region of higher concentration contain more kinetic
energy on that is why they show fast movement. During the movement of molecules
collide themselves are produce a pressure in the medium called diffusion pressure.

3. •Diffusion pressure always proportional to concentration of molecules
•Diffusion pressure always proportional to temperature
•Due to this reason, the diffusion always takes place from the region of higher DP
(conc.) to the region of lower diffusion pressure.
•-The rate of diffusion is maximum in gases and minimum in solids when dissolved
in solvent.

4. Diffusion pressure of liquids: like gases, solvents and liquids also posses diffusion
pressure. The diffusion pressure of a pure solvent is always maximum and when solute
particles are added in it. The diffusion pressure of the solution is reduced. (The differences
between the DP of a solvent and its solution is called Diffusion pressure deficit DPD.
(Water DP= 1236 atm)
When a deficiency in diffusion pressure of the solution is created, it starts absorbing more
solvent particles to overcome this deficiency.
OR
Diffusion pressure deficit – Wall pressure tends to force the water out of cell and acting
against the osmotic entry of water in the cell. Now absorption of water takes place which
depends upon difference between osmotic pressure and wall pressure. During this time wall
pressure equals turgor pressure. This difference is called diffusion pressure deficit which is
the difference between osmotic pressure and turgor pressure and can be expressed as follows:
DPD = OP-TP (WP), TP= OP-DPD, OP=TP+DPD

5. Factor affecting the rate of diffusion:
Temperature=It is directly proportional to the rate of diffusion. Or increasing the
temperature, the rate of diffusing particles is also increased because of increase in the
velocity of the particles.
Concentration of the medium- it is inversely proportional to the rate of diffusion
on increasing the concentration of the medium. The rate of diffusion is reduced and or
decreasing the concentration it is increased.
The size and mass of the diffusing particles- if the size and mass of the diffusing
particles is smaller the rate of their diffusion will always be faster.

6.  Diffusion presser gradient (DPG)- The rate of diffusion of molecules of gasses and
liquids also depends upon the diffusion pressure gradient (DPG) when the difference in the
diffusion pressure is more faster is the rate of diffusion.
 Density of the diffusing particles; the rate of diffusion of gas is related with the density
of diffusing particles. (According to grahams law of diffusion- the rate of diffusion of any
gas particles is inversal proportional to the solvare root of its density.
= The gases having higher densities show slower rate of diffusion
r = 1//d d= density of gas

7. Importance of diffusion in plants:
•Exchange O2 or CO2 gases in atmosphere
•Stomatal transpiration- water vapour- intercellular space diffuse- atmosphere
through stomata
•Ions or mineral salts- passive absorption
•Absorption of water- root

8. Permeability: The entrance or exit of any substance in the cell depends upon the
permeability of the plasma membrane. (Depending upon the permeability properties
the membrane can be classified in to following categories:
Permeable membrane- allows all substance to flow freely across the membrane.
Ex- Plant cell wall, Bacterial and fungi cell wall
Impermeable membrane- through which only exchange of gases takes place.
Ex. cuticle layer of leaf
Semi permeable membrane- Exchange only water or solvent molecule.
Ex. Parchment membrane, Urinary Bladder of goat, Egg membrane.
Selective membrane- Exchange only selected ions or small molecules. (Plasma
membrane) PM is usually permeable for gases like CO2, O2, N2.

9. Factors effecting permeability:
Physical agent: Temperature, Heat, Low pressure of O2 and CO2, radiation.
Chemical agents- Chemical like- ether, benzene, chloroform, acetone, toxic
substance etc. When added in the external medium or solution (Permeability)
Membrane constitution: the permeability of plasma membrane. Depends upon
its constitution ex. Lipids, Protein, carbohydrates and phosphate etc.

10. Osmosis: When two solutions of different concentrations are separated by semi
permeable membrane, the diffusion of water or solvent molecules takes place from
the solution of lower concentration towards the solution of higher concentration this
process is called Osmosis.
Osmotic pressure: is the pressure required to stop water from diffusing through a
membrane by osmosis.

11. Osmosis –
Osmosis is the process of diffusion of water across a semipermeable membrane.
Water will move in the direction where there is a high concentration of solute
(and hence a lower concentration of water).
Water molecules are free to pass across the cell membrane in both directions,
either in or out, and thus osmosis regulates hydration, the influx of nutrients and the
outflow of wastes, among other processes.
*A simple rule to remember is: salt sucks.
Salt is a solute, when it is concentrated inside or outside the cell, it will draw the
water in its direction. This is also why we get thirsty after eating something salty.

12. Types of Osmosis:
1.Endosmosis: when water or solvent molecules enter into the cell through plasma
membrane from the outer medium, it is called endosmosis.
2. Exosmosis: When a plant cell is placed in concentrated solution the water molecule
move from cell into the outer concentrated medium through plasma membrane it is
called Exosmosis.

13. Type of Solutions :
1. Isotonic Solutions
2. Hypotonic Solutions
3. Hypertonic Solutions
1. Isotonic Solutions: If the concentration of solute (salt) is equal on both sides
of membrane, the water will move back in forth, but it won't have any result on
the overall amount of water on either side. "ISO" means the same.

14. 2. Hypotonic Solutions The word "HYPO" means less, in this case there are less
solute (salt) molecules outside the cell, since salt sucks, water will move into the cell.
The cell will gain water and grow larger. In plant cells, the central vacuoles will fill,
and the plant becomes stiff and rigid, the cell wall keeps the plant from bursting. In
animal cells, the cell may be in danger of bursting, organelles called contractile
vacuoles will pump water out of the cell to prevent this.

15. 3. Hypertonic Solutions: The word "HYPER" means more, in this case there are more solute
(salt) molecules outside the cell, which causes the water to be sucked in that direction. In
plant cells, the central vacuole loses water and the cells shrink, causing wilting. In animal
cells, the cells also shrink. In both cases, the cell may die.
Therefore it is dangerous to drink sea water - its a myth that drinking sea water will
cause you to go insane, but people marooned at sea will speed up dehydration (and death) by
drinking sea water.
This is also why "salting fields" was a common tactic during war, it would kill the crops
in the field, thus causing food shortages.
 Both Diffusion and Osmosis are types of passive transport, that is, no energy is required
for the molecules to move into or out of the cell.
Sometimes, large molecules cannot cross the plasma membrane, and are "helped" across by
carrier proteins - this process is called facilitated diffusion.

17. Factor Affecting Osmotic pressure:
•Concentration of solute particle is more than osmotic pressure will increase.
•Temperature increase osmotic pressure will also increase.
•Ionization of solute molecule concentrated increase than osmotic pressure increase.
•Light intensity will increase than osmotic pressure increase.
•Hydration of water molecules increase than osmotic pressure increase

18. Significance of Osmosis in Plants:
Osmosis helps in the absorption of water from soil through roots.
The turgidity in plant cell--- maintained—osmosis.
Diffusion of water from one cell to another---Osmosis
Opening stomata---Depend upon turgidity
Help in the growth of young cell
Help in the dehiscence of fruit.

19. Water potential and its components : Water potential is the potential energy of
water per unit volume relative to pure water in reference conditions. Water potential
quantifies the tendency of water to move from one area to another due to osmosis,
gravity, mechanical pressure and matrix effects such as capillary action.
Water potential or chemical potential of water is a quantitative expression of the
free energy associated with water. Water potential is symbolized by the Greek letter
ψ (psi) and is defined relative to the water potential of pure water, which is zero.
Hence the value of psi is always negative. The units of water potential are mega
Pascal (MPa).

21. Osmotic potential : The osmotic potential, Ψs (or π) is the component produced by
the solute dissolved in the cell sap, chiefly vacuolar sap.
Matric Potential : The matric potential Ψm (or T) refers to water held in micro
capillaries or bound on surfaces of the cell walls and other cell components.
Pressure potential : The pressure potential Ψp (or P) is the turgor pressure produced
by diffusion of water into protoplasts enclosed in walls which resist expansion. In the
xylem of transpiring plants Ψp is usually negative and in guttating plants it is positive
as a result of root pressure.
It is a relative quantity and depends on concentration, pressure and gravity at
the same temperature. Water potential as the sum of component potentials which
may be written as
Ψ= Ψs+ Ψm+ Ψp + Ψg
Where, Ψs = Solute osmotic potential (symbol π)
Ψm = Matric potential (symbol T)
Ψp = Pressure potential (symbol P)
Ψg = Gravitational potential (symbol G)

22. Gravitational Potential :
The effect of gravity, Ψg (or G) is a term of negligible importance within root or a
leaf but becomes important in comparing potentials in leaves at different heights on
trees and in soils.
Upward movement of water in a tree trunk must overcome a gravitational force of
0.01 Mpa/m and gravity causes drainage of water downward in soil. The volume of
matric water is very small as compared to the volume of vacuolar water in
parenchyma, therefore potential water constitutes a small fraction of the total water,
matric potential can control the cell water potential. Thus, for herbaceous plants and
annual field crops of a short vertical height (less than 10 m) the values of the matric
potential and gravitational potential are small and are commonly omitted. Thus
Ψ= Ψs+ Ψp
Water always moves from less negative water potential to more negative water
potential.

23. Importance of water potential:
 Water potential is a diagnostic tool that enables the plant scientist to assign a
precise value to the water status in plant cells and tissues.
 The lower the water potential in a plant cell or tissue, the greater is its ability to
absorb water.
 Conversely, the higher the water potential, the greater is the ability of the tissue to
supply water to other more desiccated cells and tissues.
 Thus, water potential is used to measure water deficit and water stress in plant
cells and tissues.
 As a general rule, leaves of most plants rooted in well watered soils are likely to
have water potentials between about -2 to -8 bars.
 With decreasing soil moisture supply, leaf water potential will become more
negative than -8 bars and leaf growth rates will decline.
 Most plant tissues will cease growth completely (i.e., will not enlarge) when
water potential drops to about -15 bars.

24. Uptake of water: The way in which water is entered into the root hair and the
precise mechanism of water absorption can be explained by two different
approaches:
(a)Active uptake: Water is absorbed because of activities in the root itself and
does not concern with any process in shoot.
(b)Passive uptake of water: The governing force of water absorption
originates in the cells of transpiring shoots rather than in root itself.
Although the absorption of water by roots is believed to be a passive,
pressure driven process, it is nonetheless dependent on respiration. Respiratory
inhibitors (such as cyanide), anaerobic conditions (waterlogged condition) decrease
in the hydraulic conductance of most roots. These are some supporting points for
active absorption of water.
However, the exact role of respiration and active uptake is not clear.
Except for few exceptions, it is now believed that uptake of water is a Passive
process. Tension or negative pressure originating at the actively transpiring leaf
surface creates a pulling force for water movement in xylem (Cohesion-tension
theory of Dixon and Jolly).

25. The movement of water inside the plant is driven by a reduction in free energy, and water
may move by diffusion, by bulk flow or by a combination of these fundamental transport
mechanisms. Water diffuses because molecules are in a constant thermal agitation, which
tends to even out concentration differences. Water moves by bulk flow in response to a
pressure difference, whenever there is a suitable path way for bulk movement of water. Thus,
water potential difference (i.e., solute potential and pressure potential) across the cells
starting from root hairs to xylem plays an important role in uptake and transport of water.

26. METHODS OF MEASURING WATER STATUS IN PLANTS:
There are two general ways to describe the water status or internal water balance of
plant and plant tissue:
A. The first one is based on the energy associated with water in the plant tissue.
Water potential is considered by most plant physiologists to be the most useful
and significant way to describe the water status of plant tissues. In terms of water
potential, water deficit exists in a tissue whenever its water potential is less i.e.,
more negative than zero mega Pascal (Mpa).
The water potential is measured by
(1) Liquid immersion method (dye method)
(2) Vapor equilibration method (Thermocouple Psychrometer) and
(3) Pressure chamber method.

27. 1. Liquid immersion method or dye method or Chardakov’s Falling Drop
Method:
Tow graded series of sucrose solutions (ranging from 0.15 to 0.50 molal in
increments of 0.5 molality) are placed in test tubes, set up in duplicate.
Homogeneous plant tissue is placed into each test tube of one of the series (test
series). Only a drop of methylene blue is mixed into each solution of the second
series (control series).
Plant tissue is not added to the control series and the dye does not appreciably
change the osmotic potentials.
After the tissue is incubated for 15 to 30 minutes, it is removed from each tube.
The actual time of incubation can be just long enough for osmosis to proceed and
change the concentration of each solution in the test series; the attainment of
equilibrium is not necessary.

28. After the tissue is removed, a small drop of the respective control series
solutions is introduced below the surface of its corresponding test solution.
If the drop rises in the test solution, it means that the drop is lighter and that
the tissue incubation solution is more concentrated; an indication that water from
the solution entered the tissue.
Conversely, if the drop falls, it means that the test solution is lighter-an
indication that water has left the tissue and diluted the solution.
In this latter instance, the water potential of the solution initially is more
negative than that of the tissue.
Accordingly, if the density of the drop from the methylene blue solution is the
same as that of the test solution, the drop will diffuse into the solution uniformly.
At this point (called the null point), the water potential of the tissue and
solution is equal.

29. 2 Vapour equilibration (Thermocouple Psychrometer) Method
Psychrometry:
(The prefix "psychro-" comes from the Greek word psychein, "to cool") is based
on the fact that the vapor pressure of water is lowered as its water potential is reduced.
Psychrometers measure the water vapor pressure of a solution or plant sample, on
the basis of the principle that evaporation of water from a surface; cools the surface.
Investigators make a measurement by placing a piece of tissue sealed inside a
small chamber that contains a temperature sensor (in this case, a thermocouple) in
contact with a small droplet of a standard solution of known solute concentration
(known Ψs and thus known Ψw).
If the tissue has a lower water potential than that of the droplet, water evaporates
from the droplet, diffuses through the air, and is absorbed by the tissue.
This slight evaporation of water cools the drop.
The larger the difference in water potential between the tissue and the droplet, the
higher the rate of water transfer and hence the cooler the droplet.

30. If the standard solution has a lower water potential than that of the sample to be
measured, water will diffuse from the tissue to the droplet, causing warming of the droplet.
Measuring the change in temperature of the droplet for several solutions of known Ψw
makes it possible to calculate the water potential of a solution for which the net movement
of water between the droplet and the tissue would be zero signifying that the droplet and the
tissue have the same water potential.
Psychrometers can be used to measure the water potentials of both excised and intact
plant tissue. Moreover, the method can be used to measure the Ψs of solutions. This can be
particularly useful with plant tissues.
A major difficulty with this approach is the extreme sensitivity of the measurement to
temperature fluctuations. For example, a change in temperature of 0.01°C corresponds to a
change in water potential of about 0.1 MPa.
Thus, psychrometers must be operated under constant temperature conditions. For this
reason, the method is used primarily in laboratory settings.

31. 3. Pressure chamber method :
A relatively quick method for estimating the water potential of large pieces of
tissues, such as leaves and small shoots, is by use of the pressure chamber.
This method was popularized by P. Scholander and coworkers.
The pressure bomb is a device that is used to determine the plant moisture stress and
the water potential of a leafy shoot and is based on the assumption that the water
column in a plant is almost always under tension because of the pull exerted by the
osmotic influences (water potential) of the living cells of the leaves.
If the tension is high, the water potential of the leaf cells is very negative.
When a stem is cut, the water column (in xylem) is disrupted and because the water
column is under tension, it will recede back into the stem toward the leaves.

32. The shoot is placed in a chamber, with the cut end protruding through an airtight hole.
Pressure is increased within the chamber and the water column within the twig is forced
back to the cut surface.
The pressure in the chamber is then carefully recorded.
The pressure required to force the water to appear at the cut surface is equal to the tension
(but with the opposite sign) of the water column at the time the shoot was cut.
If low pressure is sufficient to force water to the cut surface of the shoot, the shoot is under
relatively low moisture stress.
But if high pressure is required to force to the cut surface the moisture stress (tension) is
relatively high due to very negative water potential of the leaf cells.

33. Thank You